This class is given to you when you migrate your database from one version to another.
It contains two properties: OldRealm and NewRealm.
The NewRealm is the one you should make sure is up to date. It will contain
models corresponding to the configuration you've supplied.
You can read from the OldRealm and access properties that have been removed from
the classes by using the dynamic API.

In order to handle manual migrations, you need to supply a migration callback to your
RealmConfiguration. It will be called with a Migration instance containing
the pre- and the post-migration Realm. You should make sure that the NewRealm
property on it contains a database that is up to date when returning. The oldSchemaVersion
parameter will tell you which SchemaVersion the user is migrating
from. They should always be migrating to the current SchemaVersion.